The contours or camber of the airfoil surface of an aircraft such as the wings on fixed wing aircraft or the rotor blades on helicopters are, of necessity, a compromise in design. On aircraft wings the camber characteristics for lift introduce drag, and the camber characteristics for forward flight result in a decrease in lift. A similar problem is present in the rotor blades of a helicopter, where the optimum camber for vertical flight is quite different from that for hovering or for forward flight. In addition, the rotor blade, in operation, produces a twisting moment along the length thereof, which, as a consequence, produces a bending moment which is greatest at the root of the blade. This bending moment produces stress and results in fatigue of the metal of the blade especially at the root.
As a consequence most aircraft wings are designed to have a camber that approaches optimum for the anticipated normal usage, such as the straight forward constant altitude flight of an airliner, for example, and alteration of the airfoil characteristics when necessary is accomplished through flaps and tabs such as at landing and during climb and especially at takeoff.
The problems introduced by a constant camber have been long recognized and apparatuses to adjust the actual camber of the lifting surface of an aircraft are disclosed in numerous prior art patents. Thus, in U.S. Pat. No. 4,741,503 of Anderson et al a camber adjusting system is disclosed which utilizes a velocity signal to increase or decrease the camber of the airfoil during flight. An on board computer programmed with a plurality of algorithms determines, on the basis of the measured velocity, angle of attack, and altitude, whether a camber adjustment is necessary, and sends an adjusting signal to an actuation device to adjust the camber towards optimum for the measured flight parameters. U.S. Pat. No. 4,899,284 of Lewis et al likewise shows an airfoil camber adjusting system which measures flight conditions such as speed, normal acceleration, and weight, and applies signals to a calculating means which utilizes the signals, along with stored data, to compute the optimum camber for the measured operating parameters and generates command signals to trailing edge and leading edge camber adjusting means. Neither the Lewis et al nor the Anderson et al patents disclose the mechanism by which the actual camber is altered, however, the prior art is replete with arrangement for alternating the camber of airfoils.
In U.S. Pat. Nos. 1,823,069 of Stroop, 3,698,668 of Cole, 4,247,066 of Frost et al, 4,341,176 of Orrison, and 4,444,368 of Andrews are shown arrangements for varying the camber of an airfoil. Without exception the camber varying arrangements are mechanical systems of extreme complexity which add weight to the aircraft, apparently necessitate unusual maintenance and/or repair, and cannot help but be expensive. In U.S. Pat. No. 4,582,278 of Ferguson there is shown a camber adjusting arrangement having an inflatable/deflatable cavity which receives fluid under pressure. Such an arrangement is, in some respects, less complex than the aforementioned prior art arrangements, but it introduces its own complexities such as the addition of a supply of fluid under pressure, and the obvious problems of leakage.
In all of the prior art arrangements, complexity, with concomitant costs and maintenance requirements, is common to all.